Bactericida de Corrosion

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LA 96188 2nd NACE Latin American Region Corrosion Congress september 1996 BACTERIAL INHIBITION OF CORROSION Héctor A.Videla Department of Chemistry, Faculty of Pure Sciences, University of La Plata, 1900 La Plata. ARGENTINA Tel./Fax: 54-21-257945 - e.mail: SUMMARY Microorganisms influence corrosion by changing the electrochemical conditions at the metal/solution interface. These changes may have different effects, ranging from the induction of localized corrosion to co
  LA 96188 2 nd NACELatin AmericanRegion CorrosionCongressseptember 1996 BACTERIAL INHIBITION OF CORROSION Héctor A.Videla Department of Chemistry, Faculty of Pure Sciences, University of La Plata,1900 La Plata. ARGENTINATel./Fax: 54-21-257945 - e.mail:  SUMMARY Microorganisms influence corrosion by changing the electrochemical conditions at themetal/solution interface. These changes may have different effects, ranging from the inductionof localized corrosion to corrosion inhibition. The key to the production of these effects at ametal surface and hence, the enhancement or inhibition of corrosion is linked to biofilmformation.Several of the mechanisms for interpreting the effect of microorganisms on corrosioninitiation and corrosion inhibition are critically discussed in this paper and later illustrated withseveral examples.Seldomly mentioned in the literature, microbial inhibition of corrosion is presented as apotentially useful tool to counteract many of the biodeterioration cases encountered in practice. Keywords:microbial corrosion, microbiologically influenced corrosion (MIC), biofilms,corrosion inhibition, extracellular polymeric substances (EPS), sulphate-reducing bacteria (SRB), Serratia marcescens. 1. INTRODUCTION Microorganisms influence corrosion by changing the electrochemical conditions at themetal/solution interface. These changes may have different effects, ranging from the induction  of localized corrosion, through a change in the rate of general corrosion, to corrosion inhibition (1). Although the electrochemical nature of corrosion remains valid for microbial corrosion,the participation of the microorganisms in the process induces several unique features, mainlythe modification of the metal/solution interface by biofilm formation. Thus, the key to thealteration of conditions at a metal surface, and hence the enhancement or inhibition of corrosion is the formation of a biofilm (2). This can be considered as a gel containing 95% or more water, made of a matrix of exopolysaccharidic substances (EPS) in which microbial cells,and inorganic detritus are suspended (3).Microbial colonization of metal surfaces through biofilms drastically changes theclassical concept of the electrical interface commonly used in inorganic corrosion. Importantchanges in the type and concentration of ions, pH values and oxidation-reduction potential areinduced by the biofilm, altering the passive or active behavior of the metallic substratum and itscorrosion products, as well as the electrochemical parameters used for assessing corrosionrates (4). 2. BIOFILM EFFECTS ON CORROSION  A practical assessment of the interaction between corrosion and biofilms was made for several metal surfaces in seawater, using scanning electron microscopy (SEM) complementedwith energy dispersive X-ray analysis (EDXA) of the deposits, and electrochemical corrosiontechniques to evaluate the corrosion behavior of the metallic susbstratum (5). According tothese results, biofilm formation at the metals surfaces tested increased in the following order:copper > 70:30 copper-nickel alloy > brass > aluminum > stainless steel > titanium. At activemetal surfaces like aluminum, biofilms are formed on an unstable and continously growing layer of inorganic products. Conversely, at corrosion-resistant surfaces like stainless steel andtitanium, a rapid and easy microbial colonization occurs on an even and stable metal surface.In the case of corrosion resistant metals (e.g. stainless steel) biofilms can stimulatecorrosion in two main ways: a) by initiating corrosion through the formation of differentialaeration cells and, b) by increasing the rate of the cathodic reaction. Generally, biofilm-metalinteractions on a corrosion-resistant alloy can lead to the initiation of localized corrosion thatwould not occur in the absence of biofilms. The initiation of corrosion through differentialaeration may occur as a result of a patchy distribution of the biofilm and/or by an alteration of oxygen gradients within the biofilm matrix. Besides, the growth of different species of microorganisms in the biofilm facilitates the development of structured consortia that usuallyenhance the effects of single microbial species on metal corrosion (6). Conversely, on an easilycorrodible surface such as carbon steel in saline media, abundant deposits of corrosionproducts of varied chemical composition rapidly cover the metal surface leading to complexbiofilm-corrosion product interactions. For instance, in marine environments a complex foulinglayer of bacteria and algae embedded in EPS generally consolidates the corrosion productlayers. This cohesive effect depends on several environmental and biological factors and willfinally determine the corrosion behavior of carbon steel.Copper-nickel alloy surfaces can be colonized by bacteria and other organisms after several weeks of exposure to marine environments, despite their perceived anti-foulingproperties. In these surfaces, biofilm formation is markedly conditioned by the chemical natureand distribution of the inorganic layers, and by the elemental composition of the substratum (7,8). Usually, bacteria can be found entrapped between corrosion product layers containing EPSleading to a layered structure. Thereafter, biofilm detachment could influence the removal of  2  inorganic passive layers, resulting in a patchy distribution of the biofilm which facilitatescorrosion by differential aeration. 3. MECHANISMS OF MICROBIAL CORROSION Classic mechanisms proposed for interpreting microbial corrosion can be summarized asfollows (9):- Metabolic production of aggressive compounds that change the environment from inert toaggressive.- Creation of differential aeration cells as a consequence of patchy biofilms on the surface.- Microbial disruption of protective coatings (or passive films).- Metabolic uptake of  corrosion inhibitors present in the medium.- Acceleration of one of the corrosion reactions by a depolarization effect (e.g. hydrogenconsumption).Several of microbial corrosion mechanisms can operate simultaneously or consecutively,but generally no single cause can account for all the corrosive effects of microorganisms.Frequently, a multiple mechanism, involving synergistic effects between microorganisms, metalsurfaces and each environment must be invoked to explain each practical case of microbialcorrosion (9). It should be stressed that one of the main effects of microorganisms in corrosionis the modification of the metal-solution interface through biofilm formation. 4. BACTERIAL INHIBITION OF CORROSION Microorganisms can help to inhibit corrosion through several mechanisms:a) by neutralizing the action of a corrosive substance present in the medium (e.g. throughmetabolic consumption).b) by stabilizing a passive film on a metal surface (e.g. through a change in redox conditions).c) inducing a decrease in the medium aggressiveness (e.g. by a favorable change of pH inrestricted areas). 3  4.1. Mechanism a): microbial inhibition by diminishing the corrosiveness of aggressivesubstances One of the most common mechanisms by which microorganisms can induce or facilitatecorrosion is the production of aggresive metabolites of acidic nature that enhance anodic metaldissolution or depolarize the cathodic reaction by providing cathodic reactants. Thus, anybiological activity that directly or indirectly counteracts those effects could inhibit corrosionwhen adequate environmental conditions are available. An example of this mechanism is related with corrosion inhibition of mild steel by one of three thermophilic bacterial species (10) tentatively identified by the authors as Bacillus sp. Inthis work it was also reported the effect of biofilms of  Deleya marina on the corrosion rate of mild steel 1010 showing a strong corrosion inhibition in both stirred and flow-through systems(Table 1). These results were supported by polarization resistance data that evidenced analmost complete passivation of the metal by the bacterium, reducing the corrosion rate by 94%.This inhibitory action can be reverted in mixed biofilms where microbial interactions usuallyenhance the rate of metabolic processes. Within bacterial consortia, species that passivatemetals in pure culture may enhance corrosion by providing a protective film for corrosivebacteria. Table1: Corrosion rates measured as polarization resistance in volts/milliamp (V/mA) of mildsteel in the presence of different bacterial cultures (from reference (10) with permission of Elsevier Science Ltd., Oxford, UK). Polarization(V/mA)Corrosionrate (milsper year)Reduction or increase incorrosion rate comparedto uninoculated controls (A) Uninoculated control (20 o C)  D. marina (B) Uninoculated control (65 o C)T1T2T34.764. are three different thermophilic isolates. Another example of microbial inhibition of corrosion by neutralization of corrosivesubstances is related to corrosion fatigue and hydrogen embrittlement in sour saline solutions.These environmental conditions are held to be particularly aggressive as a result of highconcentrations of atomic hydrogen at the crack tip, as a consequence of sulphur speciespresent poisoning the hydrogen recombination reaction at the cathodic surface (11). It isassumed by the authors that EPS and organics related to biofilms hinder dissolution anddissociation reactions as well as hinder adsorption. The greater the biological activity, thegreater is likely to be the barrier to embrittlement. This assumption was supported by resultsshowing crack growth rates consistently lower in biologically active media than in equivalentabiotic solutions. Thus, two opposite roles of bacteria would take place in these environments.First, the enhancement of corrosion fatigue by biologically produced hydrogen sulfide and 4
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